The present invention relates to a process for production of a compacted graphite iron alloy article, which is easy to machine.
Compacted Graphite Iron (CGI) is widely recognized as being an excellent material for car and truck cylinder blocks, among other applications. The increased strength, stiffness, fatigue resistance and wear resistance relative to conventional grey cast iron and the common aluminium alloys allows engine designers to increase performance while reducing weight and emissions. However, the improved properties of CGI also make it more difficult to machine.
Conventional machining operations such as milling, drilling, tapping and honing can be successfully performed with CGI, although tool life may be 10-50% lower than that achieved with grey iron. This reduced machinability is generally accepted in lieu of the higher strength castings. However, the tool wear encountered during high speed cylinder boring is not acceptable. The high speed ( greater than 600 m/min) boring of cylinder bores uses CBN (cubic boron nitride) or ceramic cutting tools. Although a typical cylinder bore is only about 85 mm in diameter and 95 mm deep, with tool feed rates of 0.20 mm/revolution, the machining tool experiences approximately 125 m of continuous contact with the metal as it spirals down the bore. During this continuous cut, the relatively higher hardness and strength of CGI causes the temperature of the tool to increase and the tool wear to become excessive. Under high speed machining conditions, the tool wear encountered with CGI is approximately twenty times higher than with conventional grey iron. Machining generally accounts for approximately 10% of the total cost of manufacturing a finished engine. Therefore, CGI alloys with improved machinability have been demanded to reduce costs. Indeed, CGI alloys with improved machinability, in particular high speed cylinder bore finishing, are required before CGI can be adopted for high volume (xe2x89xa7100 000 units per year) series production.
Beyond the superior mechanical properties, CGI is approximately 20% harder than grey cast iron when compared at equal pearlite content. Compacted graphite iron also has 1-3% elongation whereas grey iron has effectively no ductility. These factors contribute to altering the chip formation and tool wear mechanisms during machining.
JP 58-93854 discloses a vermicular graphite cast iron, including 3-4% C, 34.5% Si, Mn below 0.3%, P below 0.05%, S below 0.03% and Mg 0.005-0.030%, for use in the manufacturing of exhaust manifolds. The purpose of this composition is to meet the operational criteria of exhaust manifolds, namely elevated temperature fatigue strength and oxidation resistance. Nothing is said about the machinability characteristics of this composition.
There is currently no method available to achieve the needed tool life during high speed cylinder bore finish of CGI. In an attempt to minimize the hardness of CGI and thus improve the machinability of CGI, many development activities have investigated a 70% pearlitic/30% ferritic matrix, although this CGI matrix composition has approximately the same hardness as conventional grey iron. However, the high-speed machinability is not significantly improved relative to fully pearlite CGI. The CGI alloy of the present invention provides a means to overcome the machining problem, which currently prevents the industrial adoption of CGI engine blocks.
The problem to be solved by means of the present invention is to provide a CGI alloy which permits an improved machinability, particularly during high speed cylinder bore finishing, in terms of tool life and chip disposability, compared to conventional CGI alloys.
This problem is solved according to the invention as it is surprisingly found that alloying the CGI with higher than normal silicon contents improves the high speed machinability.
The conventional CGI alloy composition for engine block applications contains 2.0-2.5% silicon. However, at silicon contents between 2.8-4.0% (by weight) the CGI will solidify with a predominantly ferric matrix. The higher silicon content promotes graphite formation thus depleting the matrix of free carbon and preventing the eutectic formation of iron carbide (Fe3C). Additionally, in contrast to normal ferritic irons which are relatively soft and weak and tend to adhere to the cutting tool and or/tear during machining, the high silicon content results in a hard ferrite. The silicon content can be selected to achieve the same hardness range as for conventional grey iron while retaining a fully ferritic matrix. Altematively, the silicon content can be varied to achieve the desired hardness level and range. The free silicon atoms in the iron matrix harden the ferrite by a solid solution mechanism, which maintains strength and wear resistance while providing improved chip removal and improved tool life.
In brief, the composition of the CGI alloy of the present invention essentially comprises, in weight %, about: 3.2 to 3.8 total carbon C; 2.8 to 4.0 silicon Si; 0.005 to 0.025 magnesium Mg; and the balance iron Fe and incidental impurities, wherein Mg may be added separately or in combination, up to 0.025%. The unique aspects of the present invention reside in the fact that the machinability of the alloy is controlled by alloy chemistry. The CGI articles so produced achieve the desired microstructures and properties prior to machining, with no changes required to the conventional machining procedures for grey cast iron.
More preferably, the CGI alloy of the invention comprises essentially, in weight % about: 3.2 to 3.8 total carbon; 2.8 to 4.0 silicon; 0.005 to 0.025 magnesium; up to 0.030 sulphur; up to 0.4 manganese; up to 0.2 copper; trace tin and the balance iron and incidental impurities. Additions of Mg may also be made as specified above.
The silicon content can be selected to achieve the same hardness range as for conventional grey iron while retaining a fully ferric matrix, wherein the alloy comprises 2.8-4.0 weight % silicon as disclosed above.
The present invention is also directed to a process for making a compacted graphite iron (CGI) article, comprising the steps of:
(a) providing a CGI base iron comprising, in weight percentages, about
3.2 to 3.8 total C
2.8 to 4.0 Si
and the balance at least Fe and incidental impurities.
(b) treating, controlling and casting the alloy in a manner known per se,
(c) allowing the cast component to cool in the mould to a temperature of at least 775xc2x0 C., prior to shake out
(d) cleaning the casting in a manner known per se and machining the casting to produce a finished article.
The invention further relates to a machinable CGI material obtained by the following steps:
(a) providing a CGI base iron having a composition as described above,
(b) treating, controlling and casting the alloy in a manner known per se,
(c) allowing the cast component to cool in the mould to a temperature of at least 775xc2x0 C., cleaning the casting in a manner known per se.
The invention also relates to the use of the CGI alloy composition for the production of a CGI article by machining.
The machining step comprises one or more working operations selected from the group consisting of milling, drilling, tapping, honing and boring, which may be conducted with a variety of cutting materials and cutting conditions (speed, feed, depth of cut, tool geometry, tool coatings etc). Although the present CGI alloy is intended to improve all cutting operations, it is primarily effective in high speed boring and turning operations, where the cutting edge is in continuous contact with the cast alloy.
Base iron is referred to as the iron held in the furnace before Mg and inoculant is added.
The production, control and fettling of the proposed high-silicon CGI alloys (Hi-Si CGI alloys) are the same as those used for conventional CGI. The only significant difference is that additional silicon, in the form of silicon carbide or ferro-silicon or any other commercial silicon source, is added to the bath either during melting or holding of molten iron. Conventional casting methods are used and the castings are allowed to cool in the sand moulds until the bulk temperature is less than 775xc2x0 C. Thereafter the casting can be air cooled, cleaned and prepared for machining.
In order to make the invention easy to understand and produce, it will be described with reference to the appended Table I below.
Table I is a diagram of an embodiment of the alloy of the present invention which was melted and examined for chemical composition. In comparison to xe2x80x98standardxe2x80x99 predominantly pearlitic CGI alloys for engine block applications, the new high-silicon CGI has a ferritic matrix and is characterized by the following compositional differences.
The roles assumed by the various alloying components are as follows:
C: 3.2 to 3.8 weight %.
Carbon is necessary to ensure adequate graphite formation. In compacted form, the graphite particles provide good thermal conductivity and good vibration damping. The carbon content of Hi-Si CGI may be slightly reduced relative to conventional CGI to maintain a constant carbon equivalent (CE=xcex5+xcfx81i/3).
Si: 2.8 to 4.0 weight %.
Silicon is a very strong graphitizing agent promoting the precipitation of carbon and the growth of graphite particles. This ultimately results in a ferritic matrix. Silicon also increases the hardness of the ferrite phase. Silicon contents in excess of 2.8% are required to stabilize a predominantly ferritic matrix. The increased silicon content also increases the carbon equivalent (CE=xcex5+xcfx81i/3) of the iron.
S: up to 0.030 weight %.
Sulphur is a contaminant that is unavoidable in cast irons. It reacts with calcium, magnesium, rare earth metals and manganese to form harmless sulphide inclusions. The manganese sulphide inclusions improve machinability in some steels but are ineffective for this purpose in magnesium-treated cast irons.
Mg: 0.005 to 0.025 weight %.
Magnesium is intentionally added to control the growth behaviour of the graphite particles. CGI is typically stable within a range of approximately 0.008% Mg, depending on the presence of impurity elements and the cooling rate of the casting.
Mn: up to 0.4 weight %
Manganese is a common element in raw materials used to melt cast iron. It promotes pearlite formation and should therefore be reduced relative to conventional CGI. It is generally balanced with sulphur in the amount
% Mn=0.2*%S+0.18
Cu: up to 0.2 weight %
Copper is commonly added to CGI and some ductile irons to stabilize pearlite. Additions of up to 1.0% are required to establish a predominantly pearlite matrix in CGI. Lower additions are preferred for Hi-Si CGI. The reduced copper content provides a cost reduction relative to conventional CGI.
Sn: trace
Tin is a very strong pearlite stabilizer. It is typically added together with copper (1.0% Cu and 0.1% Sn) to stabilize a fully pearlite matrix in conventional CGI. Limiting tin to xe2x80x9ctracexe2x80x9d amounts assists in formation of a fully ferritic Hi-Si CGI matrix. The reduced tin content provides a cost reduction relative to conventional CGI.
Specific alloys made according to the present invention are set forth in the following examples: